1. Field of the Invention
The present invention relates generally to a long-period fiber grating filter device, and in particular, to a temperature compensating long-period fiber grating filter device which permits the use of long-period grating device without temperature control.
2. Description of the Related Art
An optical fiber grating is generally used as a filter for selecting an optical signal at a specific wavelength from multiple wavelengths propagating along a core. The optical fiber grating can eliminate or reflect light at a specific wavelength by inducing a periodic change in the refractive index of an optical fiber using an ultraviolet (UV) laser. The optical fiber grating is categorized into short period gratings and long-period gratings.
The short period gratings reflect light at a specific wavelength in the filtering process, whereas the long-period fiber gratings or transmission gratings remove light without reflection by converting the optical signal propagating in the same direction along the core mode into the cladding mode. The long-period fiber gratings includes a plurality of reflective index perturbations spaced along the fiber by a predetermined distance that ranges from several tens of .mu.m to several hundreds of .mu.m, and capable of flattening the spectral gain dependence of an EDFA (Erbium Doped Fiber Amplifier) due to its ability to remove light at the intended wavelength by coupling light from a guided mode to a non-guided mode.
The long-period fiber gratings are fabricated by varying the refractive index in a core of an optical fiber to be sensitive to UV radiation for every predetermined distance. The refractive index is increased in the core portion exposed to the UV radiation but not changed in the core portion not exposed to the UV radiation, causing a periodic change in the refractive index along the longitudinal axis of the optical fiber.
However, the long-period fiber gratings (LPGs) exhibit temperature sensitivity, and the optical characteristics are influenced by the ambient refractive index of the cladding. The long-period gratings show a high temperature sensitivity, typically in the order of 5-15 nm/100.degree. C. In order to use as a gain flattening filter, an exact and a stable shaping of the LPGs spectrum to temperature variation is essential because a small shift of the filter spectrum produces large fluctuations in the flattened gain spectrum.
As a solution for compensating the temperature sensitivity of the LPGs, we propose a novel mechanism which uses a general polymer as a recoating material.
A mathematical expression is available for describing the characteristic of an optical fiber, for example, coupling of a given wavelength occurs in a long-period fiber grating filter device when the phase matching condition of Eq. 1 is satisfied. ##EQU1##
wherein .beta..sub.co represents the propagation constant in a core mode, .beta..sub.cl.sup.(m) represents the propagation constant in an m.sup.th order of cladding mode, and .LAMBDA. represents the grating period.
If ##EQU2##
(n represents a refractive index and .lambda. represents a wavelength), the following equation follows: EQU (n.sub.co 31 n.sub.cl.sup.(m))=.lambda./.LAMBDA. (2)
As shown in equation (2), the wavelength of light at which it can be converted to a cladding mode can be determined by the grating period .LAMBDA. and the refractive index difference (n.sub.co -n.sub.cl.sup.(m)).
The refractive index difference is obtained by appropriately irradiating a UV-sensitive optical fiber with the UV light. The UV light is projected onto the amplitude masks with a specific grating period .LAMBDA.. Then, the optical fiber reacts to the UV radiation in such a way that the refractive index of a core changes and creates fiber grating. In order to obtain the intended spectrum (i.e., intended coupling wavelength and extinction ratio) from the long-period fiber grating filter device, the UV light should be projected for an appropriate time while accurately controlling the masking period.
Furthermore, the coupling wavelength of the above optical fiber gratings is temperature sensitive. Accordingly, a shift in the coupling wavelength with respect to temperature change is determined by the variations in the refractive index and lengthwise thermal expansion with temperature change. This can be expressed as follow: ##EQU3##
wherein T represents temperature.
When a long-period fiber grating filter device is fabricated of a general communication optical fiber or dispersion shifted optical fiber, ##EQU4##
is larger than ##EQU5##
by several tens of times, and thus ##EQU6##
is neglected. For example, the coupling wavelength of Flexcor 1060 of Corning shifts by 5 nm per 100.degree. C. In a typical dispersion shifted optical fiber, the coupling wavelength shifts by 0.3 nm per 100.degree. C. with respect to lengthwise expansion, and 5 nm per 100.degree. C. with respect to the refractive index change. For a real application, a temperature stability of about 0.3 nm per 100.degree. C. is required for flattening the spectral gain in a long-period optical fiber grating filter.
In prior art, in order to compensate the temperature change, the refractive index distribution in an optical fiber is designed or the grating period of the optical fiber is selected so that ##EQU7##
in Eq. 3 has a negative value. Alternatively, B.sub.2 O.sub.3 is added to the optical fiber to obtain the value of ##EQU8##
to be zero.
If .LAMBDA.&lt;100 .mu.m in the general long-period grating filter, ##EQU9##
becomes a negative value according to the conventional method of controlling the refractive index of the filter, which sets ##EQU10##
to a negative value. When .LAMBDA.=40 .mu.m, the dependence of wavelength on temperature in the Flexcor 1060 fiber is 0.15-0.45 nm/100.degree. C., but the .lambda..sup.(m) mode is in the 1.1 .mu.m region and deviates from the communication region.
A temperature compensating long-period fiber grating filter device is disclosed in detail in Korea Application No. 99-8332 entitled, "Temperature Compensating Long-period Fiber Grating Filter," filed by the present applicant.
While the recoating of the long-period fiber grating filter in above Korean co-pending application is formed of a material that would increase the refractive index with temperature increase, the refractive index of a general recoating material, such as a polymer, decreases with temperature increase due to its thermal expansion. Thus, when a general long-period fiber grating filter which shows the positive d.LAMBDA./dt is recoated with a general polymer material which shows negative dn/dt, the long wavelength shift effect by the recoating material adds to the long wavelength shift characteristic of the long-period fiber grating filter. In this case, the temperature sensitivity of long-period fiber grating filter can be suppressed with a polymer recoating which shows positive dn/dt as we mentioned in details in the Korean Application number 99-8332. Accordingly, we propose an additional technique for stabilizing the long-period grating device without temperature control.